Currently, a non-aqueous electrolyte secondary battery including a lithium ion secondary battery, which is used for a mobile device such as a mobile phone, is available as a commercial product. The non-aqueous electrolyte secondary battery generally has a constitution that a positive electrode having a positive electrode active substance or the like coated on a current collector and a negative electrode having a negative electrode active substance or the like coated on a current collector are connected to each other via an electrolyte layer in which a non-aqueous electrolyte solution or a non-electrolyte gel is maintained within a separator. According to absorption and desorption of ions such as lithium ions on an electrode active substance, charging and discharging reactions of a battery occur.
In recent years, it is desired to reduce the amount of carbon dioxide in order to cope with the global warming. As such, a non-aqueous electrolyte secondary battery having small environmental burden has been used not only for a mobile device but also for a power source device of an electric vehicle such as a hybrid vehicle (HEV), an electric vehicle (EV), or a fuel cell vehicle.
As the non-aqueous electrolyte secondary battery for application to an electric vehicle, it is required to have high output and high capacity. As a positive electrode active substance used for the positive electrode of a non-aqueous electrolyte secondary battery for an electric vehicle, a lithium cobalt composite oxide, which is a layered composite oxide, has been already widely used since it can provide high voltage at the level of 4 V and has high energy density. However, due to resource scarcity, cobalt as a raw material is expensive, and considering the possibility of having dramatic demand in future, it is not stable in terms of supply of a raw material. There is also a possibility of having an increase in the raw material cost of cobalt. Accordingly, a composite oxide having less cobalt content ratio is desired.
Similarly to the lithium cobalt composite oxide, a lithium nickel composite oxide has a layered structure but is less expensive than the lithium cobalt composite oxide. Furthermore, it is almost equivalent to the lithium cobalt composite oxide in terms of theoretical discharge capacity. From this point of view, it is expected that a lithium nickel composite oxide is used for constituting a battery with high capacity for practical use.
With regard to a lithium ion secondary battery in which a composite oxide containing lithium and nickel such as a lithium nickel composite oxide (hereinbelow, also simply referred to as the “lithium nickel-based composite oxide”) is used for a positive electrode active substance, charging and discharging are performed according to desorption and insertion of lithium ions from and to the composite oxide. At that time, since the composite oxide undergoes shrinkage and expansion in conjunction with the desorption and insertion of lithium ions, there are problems in that a great decrease in capacity occurs in accordance with repeated charge and discharge cycles by a factor such as the collapse of the crystal structure. There is also a problem in that a decrease in capacity becomes significant when the battery is used for a long period of time.
In view of the aforementioned problems, in JP 2001-85006 A, for example, a technique of forming secondary particles in a lithium nickel composite oxide with relatively large primary particles is suggested for the purpose of improving discharge capacity and cycle characteristics.